1. Field of the Invention
The present invention relates to an antenna coupling apparatus, an external-antenna connecting apparatus, and an onboard external-antenna connecting apparatus, which are suitable for electromagnetically coupling an antenna installed to a portable radio terminal and an external antenna installed to an automobile.
2. Description of the Related Art
An antenna coupling apparatus using electromagnetic coupling, called a coupler, has widely been used for onboard antenna kits to couple the antenna installed to a portable radio terminal, such as a portable phone, with an external antenna installed to an automobile.
FIG. 1 shows an example of the antenna coupling apparatus. An antenna coupling apparatus 1 comprises an electromagnetic coupling element 2 and a transmission cable 3. The electromagnetic coupling element 2 of the antenna coupling apparatus 1, which element is ring-shaped, is connected to the core of the transmission cable 3 (in some cases, the element is connected to a ground conductor 4 through a nonconducting member as shown in FIG. 1). Inserting the antenna 6 fitted with a nonconducting cover and disposed on a portable radio terminal 5 into the ring-shaped electromagnetic coupling element 2 allows the power received from an external antenna 2′ to be transmitted through the electromagnetic coupling element 2 to the portable radio terminal 5 and the power transmitted by the portable radio terminal 5 to be transferred to the external antenna without contact with respect to DC components.
FIG. 2 shows an example of an external-antenna connecting apparatus connecting an external antenna and the antenna of a portable radio terminal together. The external-antenna connecting apparatus 10 comprises a curled cord 11 and a connector 12 that connect the external antenna and an external power supply (not shown). Connecting the connector 12 of the antenna connecting apparatus 10 to a connector terminal 14 in the lower part of a portable radio terminal 13 allows the terminal 13 to receive power from the external power supply and transmit a radio frequency (RF) signal to, or receive it from, the external antenna through the curled cord 11 and the connector 12.
As shown in FIG. 3, an external antenna connecting apparatus 15 comprises an electromagnetic coupling circuit 18, fitted with a nonconducting cover, and a transmission cable 19. The electromagnetic coupling circuit 18 is provided with a hole corresponding to the antenna 17 to install the antenna 17, fitted with a nonconducting cover, of a portable radio terminal 16. For the external-antenna connecting apparatus 15, fitting the electromagnetic coupling circuit 18 over the antenna 17 in the direction indicated by an arrow allows the antenna 17 and the electromagnetic coupling circuit 18 to be electromagnetically coupled together and thus an RF signal to be transmitted to, or received from, the external antenna.
An antenna coupling apparatus incorporating an electromagnetic coupling element electrically connects a portable radio terminal and an external antenna together, using electromagnetic coupling.
It is difficult for the conventional antenna coupling apparatus 1, of FIG. 1 to receive all the power emitted from the antenna 6, using only the electromagnetic coupling element 2, so that unnecessary power emissions occur. Thus the apparatus 1 poses a problem of heavy loss which occurs when power is transmitted.
In the external-antenna connecting apparatus 10 of FIG. 2, a cable for transmitting an RF signal from an antenna power supply (not shown) installed on the portable radio terminal to the connector terminal 14 must be installed in the portable radio terminal 13 to enable an RF signal to be transmitted to, or received from, the external antenna.
The cable however, may cause the performance of the portable radio terminal 13 to be deteriorated when the terminal 13 is used independently. What is worse, installing the cable inside the portable radio terminal may causes the portable radio terminal to be difficult to reduce in size.
The external-antenna connecting apparatus 15 in FIG. 3 has the disadvantage in that the electromagnetic coupling circuit 18 is not easy to fit over the antenna 17 of the portable radio terminal 16.
Poor adhesion between the electromagnetic coupling circuit 18 and the antenna 17 causes the circuit 18 to shift, resulting in an unstable coupling with the antenna 17.
Thus increasing adhesion between the electromagnetic coupling circuit 18 and the antenna 17 causes the portable radio terminal 16 to be more frequently used by a user, so that the antenna 17 is more often installed in, or removed from, the electromagnetic connecting circuit. Since the nonconducting covers for the antenna 17 and the electromagnetic coupling circuits 18 are overloaded due to friction, their durability may be lowered. If the antenna 17 is of a threaded type, it may come off the body of the portable radio terminal 16 as the antenna connecting apparatus 15 is installed or removed.